Windows still may not have UNIX-levels of stability, but 2008 does appear to actually be stable. At last. (I don't have enough data to judge this).

Active Directory is the de facto SSO/Directory mechanism for many enterprises. It works.

The Linux desktop

I don't like KDE - it's brilliant "under the hood" but isn't usable.

GNOME is usable, simple and attractive. It would like to be written in C# and run on the Mono CLR, but there's no way a bunch of Free Software advocates are going to do that. Expect a fork and both versions to die slowly. KDE wins (and usability suffers).